Spin transport study in spintronic nanodevices

SPIN TRANSPORT STUDY IN SPINTRONICS NANODEVICES BALA KUMAR A/L SUNDARAM PILLAY (B. Eng (Hons.), National University of Singapore) REPORT SUBMITTED FOR THE DEGREE OF DOCTORATE ENGINEERING DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2008 Acknowledgements I would like to thank my supervisor Dr. Mansoor Abdul Jalil for providing guidance and suitable references that have started me into the area of spintronics. I would also like to thank my co-supervisor, Dr. Tan Seng Ghee and Dr. Teo Kie Leong for helping me in my research work. Dr. Tan Seng Ghee has been especially helpful in guiding me to solve many theoretical problems during my research work. I also would like to thank Dr. Liang Gengchiau for helping me with theoretical understanding particularly in the section of microscopic transport. S. Bala kumar ii List of Publications 1. ! S. G. Tan, M. B. A. Jalil, S. Bala Kumar, Spin tunneling in multilayer spintronic devices, Physical Review B 77, 085424 (2008) 2. ! S. Bala Kumar, S. G. Tan, M. B. A. Jalil, Bias current effects on the magnetoresistance of a FM-SC-FM trilayer, Appl. Phys. Lett. 90, 142106 (2007). 3. ! S. Bala Kumar, M. B. A. Jalil, S. G. Tan, Spin-Polarized Resonant Transport in Hybrid FM-2DEG Structure, Phys. Rev. B 75, 155309 (2007). 4. S. Bala Kumar, S. G. Tan, M. B. A. Jalil, The effect of capping layer on the spin accumulation and magnetoresistance of a CPP spin valve, Appl. Phys. Lett. 90, 163101 (2007). 5. ! S. Bala Kumar, S. G. Tan, M. B. A. Jalil et al., Nanopillar ferromagnetic nanostructure as highly efficient spin injector into semiconductor, Appl. Phys. Lett. 91, 142110 (2007). 6. ! S. Bala Kumar, S. G. Tan and M. B. A. Jalil et al., Nanoelectronic Logic Device based on the Manipulation of Magnetic and Electric Barriers, J. Appl. Phys. 103, 054310 (2008). 7. ! S. Bala Kumar, S. G. Tan, M. B. A. Jalil et al., Spin transfer torque in currentperpendicular-to-plane multilayer structure induced by spin relaxation in the capping layer, J. Appl. Phys. 103, 07A712 (2008). 8. N. L. Chung, M. B. A. Jalil, S. G. Tan, and S. Bala Kumar, Interfacial resistance and spin flip effects on the magnetoresistance of a current-perpendicular to plane spin valve, J. Appl. Phys. 103, 07F308 (2008). 9. S. Bala Kumar, S. G. Tan, M. B. A. Jalil et al., Spin Injection due to interfacial spin asymmetry in a ferromagnet-semiconductor hybrid structure, J. Appl. Phys. 102, 084310 (2007). iii List of Publications iv 10. S. G. Tan, M. B. A. Jalil, S. Bala Kumar et al., Theoretical modeling of Half-metallic CPP Spin Valves , J. Appl. Phys. 101, 09J502 (2007). 11. S. G. Tan, M. B. A. Jalil, and S. Bala Kumar, Influence of Spin Relaxation on Magnetoresistance, J. Appl. Phys. 101, 044303 (2007). 12. S. Bala Kumar, S. G. Tan, M. B. A. Jalil, Effect of Interfacial Spin Flip and Momemtum Scatering on Magnetoresistance, IEEE Trans. Magn. 43, 2863 (2007). 13. Z. Y. Leong, S. G. Tan, M. B. A. Jalil, S. Bala Kumar et al., Magnetoresistance modulation due to interfacial conductance of current perpendicular-to-plane spin valves, J. Magn. Magn. Mater. 310, e635 (2007). 14. ! S. Bala Kumar, M. B. A. Jalil, S.G. Tan et al., Magnetoresistance effects arising from interfacial resistance in a current-perpendicular-to-plane spin-valve trilayer, Phys. Rev. B 74, 184426 (2006). 15. M. B. A. Jalil, S. G. Tan, S. Bala Kumar et al., Spin drift diffusion studies of magnetoresistance effects in current-perpendicular-to-plane spin valves with half-metallic insertions, Phys. Rev. B 73, 134417 (2006). 16. S. G. Tan, M. B. A. Jalil, S. Bala Kumar et al., Layer thickness effect on the magnetoresistance of a current-perpendicular-to-plane spin valve, J. Appl. Phys. 100, 063703 (2006). 17. ! S. T. Bae, S. G. Tan, M. B. A. Jalil, S. Bala Kumar et al., Magnetoresistive behavior of current-perpendicular-to-plane trilayer with half-metal insertions, J. Appl. Phys. 99, 08T107 (2006). 18. S. G. Tan, M. B. A. Jalil, S. Bala Kumar et al., Utilization of magneto-electric potential in ballistic nano devices, J. Appl Phys. 99, 084305 (2006). 19. ∗ S. Bala Kumar, M. B. A. Jalil, S. G. Tan et al., The effect of spreading resistance on the magnetoresistance of current-perpendicular-to-plane spin valves with patterned spacer layers, IEEE Trans. Magn. 42, 3788 (2006). 20. S. Bala Kumar, S. G. Tan, M. B. A. Jalil et al., MR Enhancement in Current-Perpendicularto-Plane Spin-valve by Insertion of a Ferromagnetic Layer within the Spacer Layer, IEEE Trans. Magn. 42, 2459 (2006). 21. S. G. Tan, M. B. A. Jalil, S. Bala Kumar , Electrical control of ballistic spin-dependent conductance through magneto-electric barriers in the 2D-electron gas of GaAs heterostructure, IEEE Trans. Magn. 42, 2673 (2006). Citation Report: Total citation : 44 Non-self citation: 13 List of Publications ! Highlighted in Virtual Journal of Nanoscience and Technology ∗ Appear as the cover page of the issue. v Contents Acknowledgements ii List of Publications iii Summary x List of Tables xiii List of Figures xiv List of Symbols and Abbreviations 3 6 Introduction 1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 Electron Spin . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.2 Spintronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Spin Transport Phenomena . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Spin Generation . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1.1 Spin in Ferromagnetic (FM) and Nonmagnetic (NM) Materials . . . . . . . . . . . . . . . . . . . . . . . 1.2.1.2 Spin in Semiconductor (SC) . . . . . . . . . . . . . . 1.2.2 Spin Transport . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2.1 Transport Regime . . . . . . . . . . . . . . . . . . . 1.2.2.2 Spin Injection (SI) . . . . . . . . . . . . . . . . . . . 1.2.2.3 Spin Accumulation . . . . . . . . . . . . . . . . . . 1.2.2.4 Spin Relaxation . . . . . . . . . . . . . . . . . . . . 1.2.3 Spin Manipulation . . . . . . . . . . . . . . . . . . . . . . . . 1.2.4 Spin Detection . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Magnetoresistive Devices . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Giant Magnetoresistance (GMR) . . . . . . . . . . . . . . . . . vi 9 10 10 11 11 12 12 13 Contents 1.4 1.5 1.3.2 Magnetic Tunnel Junction (MTJ) . . . . . . . . . . 1.3.3 Spin Valve (SV) . . . . . . . . . . . . . . . . . . . 1.3.4 Magnetoresistive Random Access Memory(MRAM) Motivations and Objectives . . . . . . . . . . . . . . . . . . Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii . . . . . . . . . . . . . . . . . . . . Physics of the Trilayer CPP Structure 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Boltzmann Spin-Drift-Diffusive (SDD) model . . . . . . . . 2.2.2 Spin Accumulation, ∆µ(x) . . . . . . . . . . . . . . . . . . 2.2.3 Spin-dependent Current Density, j↑,↓ (x) . . . . . . . . . . . 2.2.4 Electrochemical Potential, µ(x) . . . . . . . . . . . . . . . 2.2.5 Magnetoresistance, MR . . . . . . . . . . . . . . . . . . . 2.3 Influence of Device Parameters on MR . . . . . . . . . . . . . . . 2.3.1 Effects of Resistivity on MR optimization . . . . . . . . . 2.3.2 Effects of Conduction Polarization on MR optimization . . 2.3.3 Effects of Spin Diffusion Length (SDL) on MR optimization 2.3.3.1 Results and Discussion . . . . . . . . . . . . . . 2.3.3.2 Conclusion . . . . . . . . . . . . . . . . . . . . . 2.3.4 Layer Thickness . . . . . . . . . . . . . . . . . . . . . . . 2.3.4.1 Results and Discussion . . . . . . . . . . . . . . 2.3.4.2 Conclusion . . . . . . . . . . . . . . . . . . . . . 2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Resistance Competitive Effect 3.1 Effect of Interfacial Resistance . . . . . . . . . . . . . . . . 3.1.1 Model I: Without Interfacial Spin-flip . . . . . . . . 3.1.1.1 Theory . . . . . . . . . . . . . . . . . . . 3.1.1.2 Result and Discussion . . . . . . . . . . . 3.1.1.2.1 Infinite Spin-Relaxation Length . 3.1.1.2.2 Finite Spin-Relaxation Length . 3.1.1.3 Conclusion . . . . . . . . . . . . . . . . . 3.1.2 Model II: Finite Interfacial Spin-flip . . . . . . . . . 3.1.2.1 Theory . . . . . . . . . . . . . . . . . . . 3.1.2.2 Result and Discussion . . . . . . . . . . . 3.1.2.2.1 Interfacial Momentum Scattering 3.1.2.2.2 Interfacial Spin-Flip Scattering . 3.1.2.3 Conclusion . . . . . . . . . . . . . . . . . 3.2 Effect of Layer Insertion . . . . . . . . . . . . . . . . . . . 3.2.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 17 18 18 19 . . . . . . . . . . . . . . . . . 23 23 26 26 27 29 29 31 32 33 34 35 37 43 44 45 48 49 . . . . . . . . . . . . . . . . 51 51 53 53 54 54 59 60 61 61 62 62 64 65 66 66 67 Contents 3.3 viii 3.2.2.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Current Confinement Effects 4.1 Effect of Spreading Resistance on Magnetoresistance . . . . . 4.1.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2 Results and Discussion . . . . . . . . . . . . . . . . . 4.1.2.1 Current Confinement . . . . . . . . . . . . 4.1.2.2 Magnetoresistance and Spreading Resistance 4.1.2.2.1 Trilayer Structure . . . . . . . . . 4.1.2.2.2 Pentalayer Structure . . . . . . . . 4.1.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . 4.2 High Spin injection with nanopillar FM nanostruture . . . . . 4.2.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Results and Discussion . . . . . . . . . . . . . . . . . 4.2.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . 4.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 70 . . . . . . . . . . . . . 71 71 73 74 74 74 74 77 78 79 80 81 85 85 . . . . . . . . . . . . 86 86 89 89 92 95 97 101 102 103 106 111 111 Introduction to Green’s Function 6.1 Mesoscopic Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Electron Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Macrosopic (Top-Down) View . . . . . . . . . . . . . . . . . . 6.2.2 Microscopic (Bottom-Up) View . . . . . . . . . . . . . . . . . 6.2.2.1 Electron as Particle . . . . . . . . . . . . . . . . . . 6.2.2.2 Electron as wave (Quantum Regime) . . . . . . . . . 6.2.2.2.1 Wave function (WF) . . . . . . . . . . . . . 6.2.2.2.2 Non Equilibrium Green’s Function (NEGF) 113 113 114 114 114 114 118 118 121 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oscillatory MR due to Resonant Tunneling Effect 5.1 Resonant Tunneling in Diffusive-Ballistic-Diffusive Regime . . . . 5.1.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1.1 Spin drift-diffusive transport in the FM electrodes 5.1.1.2 Ballistic transport model within the 2DEG . . . . 5.1.1.3 Ballistic-Diffusive Self-consistent approach . . . 5.1.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . 5.1.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Active MR device . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . 5.2.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Contents 6.3 6.4 ix Tight Binding Greens Function formulation for a mesoscopic system with magnetic and electric barriers . . . . . . . . . . . . . . . . . . . . 6.3.1 Matrix Representation of Hamiltonian . . . . . . . . . . . . . . 6.3.2 Green’s Function and Self-Energy . . . . . . . . . . . . . . . . 6.3.3 Spin Dependent Transmission Probability and Current . . . . . 6.3.4 Conductance at zero bias and zero temperature . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ballistic Spin Transport across Magnetic-Electric Barriers 7.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Results and Discussion . . . . . . . . . . . . . . . . . . 7.3.1 Effective Potential Barrier, Ueff . . . . . . . . . . 7.3.2 Number of FM gates, M . . . . . . . . . . . . . 7.3.3 Conduction channel length, d . . . . . . . . . . 7.3.4 Temperature T . . . . . . . . . . . . . . . . . . 7.3.5 Bias Voltage Vb . . . . . . . . . . . . . . . . . . 7.3.6 Magnetic Barrier Profile . . . . . . . . . . . . . 7.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . Multiscale Spin Tunneling Theory 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . 8.2 Model and Theory . . . . . . . . . . . . . . . . . . . 8.2.1 Self-consistent Model . . . . . . . . . . . . 8.2.2 Green’s Function (GF) formalism . . . . . . 8.2.3 Boltzmann spin-drift-diffusive (SDD) model 8.3 Results and Discussion . . . . . . . . . . . . . . . . 8.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 123 125 127 128 129 . . . . . . . . . . 130 130 132 134 134 135 136 137 139 140 142 . . . . . . . 143 143 145 145 147 149 150 153 Conclusion 155 9.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 9.2 Further work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 A Mathematica Code for Spin Drift Diffusion Model A Bibliography A B ANSYS Software Package for Finite Element Poison Solver B C Matlab Code for Green’s Function Formulation C Summary A detail understanding of the physics of spin transport phenomena is essential to enhance the performance of present spintronic devices, as well as in designing new devices for future applications. This thesis consists of theoretical study and simulation on the physics of spin transport in spintronic nanodevices. The spin transport phenomenon is mainly studied based on the i) semi-classical spin-drift-diffusion (SDD) equation, and the ii) mesoscopic Green’s function (GF) formalism. SDD is a phenomenological model which describes the electron transport in the presence of spin relaxation in the diffusive transport regime. GF is a quantum theoretic model of electron transport in complex and inhomogeneous systems in the mesoscopic size range. The aim of our simulation is to harness the physics of spin transport to improve the performance of devices such as the spin valves (SV) and spin-transistors, as well as to propose new design for these devices. In this thesis, first the effects of various device parameters on spin transport is analyzed in detail. Focus is given to the understanding of the fundamental physics of spin transport as well as identifying any anomalous and novel x 9.1 Conclusion 157 In summary, in this thesis we have successfully achieved the main objectives: 1. We have developed mathematical models to describe the physics of spin transport in spintronic nanodevices. The two main mathematical models that were developed in this thesis are, i.e.1) semi-classical SDD model, and 2)the mesoscopic GF formalism. 2. Using these models, we have studied the effects of various device parameters on spin transport. Via our studies, we have understood the physics of spin transport, as well as identified various anomalous and novel effects. The spin transport physics/effect that were noticed and/or studied are: • The effect of structural and physical parameters. • The effect of spin-independent resistivity on spin-asymmetry. • Anomalous MR suppression effect due to the coupling of spin relaxation with resistivity. • The complex interplay between spin-asymmetry, spin relaxation and the anomalous MR suppression effect due to increase in FM layer thickness. • competitive resistance effect due to interfacial resistance and additional layers. • Current confinement, current crowding and spread resistance due to patterning of layers. • Resonant spin tunnelling in ballistic regime. • Gate controlled MR in spin-transistors. • Ballistic spin tunneling across magnetic-electric barriers. • Effect of interfacial barrier geometry and shape. 3. By careful utilization of the results and theoretical knowledge obtained from our analysis, we further explored different means as well as proposed new designs to enhance the performance of spintronic devices. Our results, models and the simulation programs that have been developed to model spin transport are also useful for the experimentalist to predict the device performance prior 9.2 Further work 158 to conducting experiments and practical realization. 9.2 Further work The GF formulism introduced in this thesis is a very powerful tool to study the quantum transport phenomena in nanodevices. Thus by using the GF method, we can continue to study many other spin transport phenomena, which could be useful in optimizing the performance of spintronic devices. In the future I will use the NEGF method to study physics of electron transport in 1D materials, especially in graphene nanoribbons (GNR). The GNR is a quasi-one dimensional system which has attracted various interesting studies on transport,164–166 magnetic,167–173 and optical172, 173 properties. I will develop a mathematical model based on the pi-orbital tight binding (TB) method [4] to describe the electron transport in GNR. Using this model, I will investigate the effects of atomic disorder on the electronic transport in graphene. The effect of applied magnetic field in a disordered graphene will also be investigated. I will further 1. include other physical phenomena such as spin scattering and phonon scattering into the existing model; 2. model different structure such as shaped modified GNR and bilayer GNR; and investigating their effects on the electronic transport. My aim is to gain a clear understanding of these phenomena and further utilize the effects in device application, e.g. in magnetoresistive devices. 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[172] K. Wakabayashi, M. Fujita, H. Ajiki, and M. Sigrist, Phys. Rev. B 59, 8271 (1999). [173] Y.C. Huang, C.P. Chang, M.F. Lin, Nanotechnology 18, 495401 (2007). Name: Bala kumar a/l Sundaram Pillay Degree: The Degree of Doctorate Engineering Department: Electrical and Computer Engineering University: National university of Singapore Thesis Title: Spin Transport study in Spintronic Nanodevices Abstract A detail understanding of the physics of spin transport phenomena is essential to enhance the performance of present spintronic devices, as well as in designing new devices for future applications. This thesis consists of theoretical study and simulation on the physics of spin transport in spintronic nanodevices. The aim of our simulation is to harness the physics of spin transport to improve the performance of devices such as the spin-valves and spin-transistors, as well as to propose new design for these devices. In this thesis, first the effects of various device parameters on spin transport is analyzed in detail. Focus is given to the understanding of the fundamental physics of spin transport as well as identifying any anomalous and novel effects. Once transport physics and the various transport effects are well understood, then we utilize this understanding to enhance the performance of the devices. We also explore new methods and device designs in order to further improve the performance. Keywords: Spintronics, Spin valve, Gaint Magnetoresistance, spin injection, spin drift diffusion, Green’s Function SPIN TRANSPORT STUDY IN SPINTRONICS NANODEVICES BALA KUMAR A/L SUNDARAM PILLAY NATIONAL UNIVERSITY OF SINGAPORE 2008 SPIN TRANSPORT STUDY IN SPINTRONIC NANODEVICES BALA KUMAR A/L SUNDARAM PILLAY 2008 [...]... Therefore FM can be used as spin- polarizer in spintronics circuits 1.2 Spin Transport Phenomena 1.2.1.2 8 Spin in Semiconductor (SC) Although a significant amount of spin polarization arises in FM metals, this is inadequate for spin- based applications Hence, non-equilibrium spin must be introduced in semiconductor (SC) to make advanced spin- based devices SC based spintronics4 can combine the well-known advantages... spin polarized current Spin polarized current refers to the current in which electrons with one type of spin (majority spin) are significantly more than the other type of spin (minority spin) , hence there is an imbalance between spin- up and spin- down electrons 1.1 Background 5 The system that produces spin polarized current is called spin injector/polarizer The next requirement is to transport, maintain... areas: 1) generating, 2) transporting, 3) manipulating, and 4) detecting spin polarized current in solid state devices 1.2.1 Spin Generation 1.2.1.1 Spin in Ferromagnetic (FM) and Nonmagnetic (NM) Materials In ferromagnetic (FM) materials22, 23 the simplest spin transport model is the two-channel model In the two-channel model, electron transport is described as follows: a) spin- up 1.2 Spin Transport Phenomena... theoretical transport methodology of the transport regime applicable in the device or in a given experimental system Compared to charge transport, in spin transport, spin coherence is maintained for much larger time (and length) scale Figure 1.2 shows various electron transport regimes and the physical phenomena related to these regimes 1.2 Spin Transport Phenomena 1.2.2.2 10 Spin Injection (SI) Spin injection... 147 8.3 Left axis: Spin- up and down resistances as a function of interfacial barrier height U The difference of resistances becomes increasingly more divergent with U Right axis: Tunneling spin injection ratio γ increases with U due to the increasingly spin asymmetric resistances Inset: Spin- up and down resistances (left axis) and spin injection ratio (right axis) as a function of spin asymmetry η of... predetermined by reservoirs at the boundaries Ballistic transport is described by using Landauer formula Quantum transport occurs when L < λF In this regime electrons exhibit wave property 1.2.2 Spin Transport 1.2.2.1 Transport Regime Spin- polarized electron transport will occur naturally in any material in which there is a difference in the spin- populations at the Fermi level In general, spin transport. .. spin- angular momentum of Sz = + 1 and spin- down” with 2 Sz = − 1 2 1.1.2 Spintronics In conventional electronic devices, the spin property of electrons has not been utilized for any practical purposes Spintronics or magnetoelectronics,4–9 is an emerging technology which exploits the spin property of the electrons in addition to the “charge” property As as result, spintronics enables us to combine... nonequilibrium spin in the spintronic devices The system which is sensitive to this signal is called a spin- detector One of the ways to detect spin in spin- transistors is by putting a FM filter in front of the device (at the drain of the transistor) such that the filter will act as a spin sensitive detector.15 Magnetic Force Microscopy is used to image the spin state of surfaces with high resolution In semiconductors,... effect can be explained by using Julliere’s model,11 which is based on two assumptions: 1) spin of electrons is conserved in the tunneling process, and 2) tunneling of up -spin and down -spin electrons are two independent processes Based on these assumptions, spin- dependent-tunneling (SDT) – electrons originating from spinup(down) state of the first FM layer can only tunnel to the unfilled spin- up(down) states... communication Spintronics also had successfully given rise to devices for memory/data storage application, e.g spin valve and Magnetic Random Access Memory (MRAM) 1.2 Spin Transport Phenomena Although the field of spintronics looks promising, many technical issues have to be resolved in order to have successful incorporation of spins into existing technology These technical issues can be sub-divided into 4 main . of theoretical study and simulation on the physics of spin transport in spintronic nanodevices. The spin transport phenomenon is mainly studied based on the i) semi-classical spin- drift-diffusion. ratio γ increases with U due to the increasingly spin asymmetric resistances. Inset: Spin- up and down resistances (left axis) and spin injection ratio (right axis) as a function of spin asymmetry η. SPIN TRANSPORT STUDY IN SPINTRONICS NANODEVICES BALA KUMAR A/L SUNDARAM PILLAY (B. Eng (Hons.), National University of Singapore) REPORT SUBMITTED FOR THE DEGREE OF DOCTORATE ENGINEERING DEPARTMENT